Cut the onion in half and slice thinly each part .In a medium sauté pan add the olive oil and heat untill very hot.

Add the onions and sauté for a couple of minutes, add 1/2 cup of water and let it slowly evaporate, this way the onions will become very tender. Once the water has evaporate, add the Modena balsamic vinegar and the brown sugar. Cook for an additional minute, remove from the heat and set aside.

On the grill over high heat set the fillet. Cook between 5 and 7 minutes per side for rare up to 10 per side for medium. Check the doneness of the meat by pressing on it.

Let the fillet rest for a couple of minutes.

Serve the fillet with the balsamic caramelized onions and a grated orange zest on the side.